In the prior-art friction feeders of this general type, sheet decollation, during which the actually lowermost sheet of the sheet stack is delivered through the decollating gap, is turned on and off only by switching the drive of the feed roller and of the conveyor belts on and off. Therefore, undefined trailing movements of the delivery means participating in sheet decollation or sheet feeding may occur during switching off in the case of very high delivery speeds, so that the lowermost paper sheet of the sheet stack will no longer assume its starting position in the area of the sheet stack after completion of the delivery movement and achieved standstill of the delivery means, but it will come to a stop somewhere between the decollating roller and the feed roller. This may lead to disturbances in the function of the friction feeder on restart, especially if the paper-processing machine arranged downstream of the friction feeder is adjusted to and dependent on a defined synchronism with the decollation processes of the friction feeder.
Another shortcoming of the prior-art friction feeders is the fact that their delivery speed and consequently also their work performance are limited to a certain, relatively low maximum, and the individual paper sheets must have a defined minimum thickness or surface quality in order to be able to be decollated in a trouble-free manner.
Friction feeders (GB 1,234,629) have also become known, in which a continuously rotating feed roller arranged under the decollation plane is vertically movable in order to be lowered, on interruption of the decollation process, in the downward direction from the decollation plane and consequently also from the lowermost sheet of the sheet stack in order to eliminate contact. The decollating gap in this prior-art device is formed by two stationary parts arranged behind the feed roller in the delivery direction, one of which parts forms a horizontal sliding surfaces and the other forms the vertical delimiting wall of a stack shaft, wherein the distance between the lower edge of the delimiting wall and the delivery surface corresponds to the thickness of one sheet. As in the other prior-art friction feeders, the sheet stack lies on the feed roller only with its own weight in this prior-art device as well, so that reliable decollation is no longer guaranteed below a certain minimum weight of the sheet stack.
This is also true of two other prior-art sheet decollating devices (OS-PS 3,761,079 and SU-PS 913,422), in which a friction lining is provided for lifting off the sheet stack from a feed roller, or, for lifting off the sheet stack from a suction roller, lifting devices in the form of rocking levers or pivotably mounted fingers are provided, which are pivoted upward to interrupt the decollation process and, to restart the decollation process, they are lowered to the extent that the sheet stack will again lie, with the actually lowermost sheet, on the feed roller or suction roller.
Aside from the above-mentioned disadvantage, these prior-art decollation devices are also unsuitable for reaching high decollation speeds and consequently high work outputs, even though the decollation process can be switched on and off relatively accurately by these lifting devices.